Plaster, also known as plaster-of-paris, calcined gypsum or gypsum plaster, is generally assumed to be a form of calcium sulfate hemihydrate, CaSO.sub.4 .multidot.1/2H.sub.2 O. Gypsum is generally assumed to be calcium sulfate dihydrate, CaSO.sub.4 .multidot.2H.sub.2 O. The process of wetting plaster and its transformation to gypsum is known as setting. Concrete, which contains plaster, also is known to set. Gypsum cements may also be used or mixed with plasters; these cements have an intermediate amount of hydration.
Before this invention the setting time of plaster to concrete could be controlled only in the short term, from about three minutes to about 20 hours, as disclosed in U.S. Gypsum Co. brochure "What Gypsum Plaster Can Do for You", revised Aug. 1983. Such short setting times and such inability to extend them obviates the continuous use of plaster slurries in industrial processing, obviates transportation of slurries from place to place, and drastically curtails impregnation of woven or nonwoven substrates.
Closed-cell gypsum compositions for use as a thermal insulation material are disclosed in U.S. Pat. Nos. 4,240,839 and 4,161,855, but these products are closed cell, do not disclose long setting times, nor impregnation of woven textiles or nonwoven felts to make flexible gypsum material. Other patents disclosing closed-cell gypsum and short setting times for application in the field of insulation are the following patents:
______________________________________ U.S. Pat. No. Inventors ______________________________________ 1,722,648 Upson 1,798,609 Knowlton 2,593,008 Chappell 2,556,031 Dickey et al 2,602,759 Mollo 2,664,406 Armstrong 2,731,377 Riddell et al 2,862,829 Dixon et al 2,902,998 Durondeaux 2,913,346 Hoffman 2,915,301 Selden 2,921,862 Sucetti 2,979,415 Taylor 3,526,685 Foster 3,563,777 Pratt et al 3,719,513 Bragg et al 3,758,319 Ergene 3,839,059 Rothfelder et al 3,926,650 Lange et al 3,974,024 Yano 4,084,980 Motoki 4,133,638 Healey 4,229,223 Flake 4,265,964 Burkhart ______________________________________
In addition to closed-cell gypsum many other materials have been employed as insulating material in residential housing such as fiberglass, rock-wool, foamed polystyrene, polyurethane foam, and urea-formaldehyde cellulosic composites. The last three named organics suffer from having inappropriate fire wall, flame propagation, and smoke generating characteristics even though they may have suitable thermal insulating properties.
Furthermore, although the inventors here are aware of polyamideimide and asbestos fabric material for use as fire cloths, protective clothing, and the like, hitherto there has been no satisfactory method for manufacturing on a continuous basis lightweight, foamed gypsum fabrics.